1. Field of the Invention
The present invention relates to a video signal correcting device of an electronic camera suitable for correcting a dark current and a dark-time white spot included in a video signal of an electronic camera.
2. Description of the Related Art
Generally, performing long time-in-second shooting with an electronic camera causes a situation in which a subject cannot be shot correctly due to the influence of dark currents and dark-time white spots. In other words, a video signal output from an image sensor of an electronic camera generally contains dark currents and dark-time white spots. As a result, it is not possible to correctly shoot a subject.
Here, a dark-time white spot is a white spot appearing in a video signal output from an image sensor when there is a defect (contamination of impurities etc.) in a photodiode constituting the image sensor. Due to the defect, charges are generated, even though no light is irradiated thereto. The dark-time white spot occurs in parts of many photodiodes constituting the image sensor.
A dark current level and a white spot level of the dark-time white spot (magnitude of a signal) change according to the shooting temperature and the shooting time-in-second (exposure time). In other words, the higher the temperature is and the longer the exposure time is, the greater the levels become.
Further, the higher the temperature is and the longer the shooting time-in-second is, the larger the number of dark-time white spots gets. This is because the higher the temperature is and the longer the shooting time-in-second is, the larger the number of charges generated by the defect of the photodiode gets. As a result, the charges generated by the defect of the photodiode easily exceed the threshold value recognized as a dark-time white spot in the video signal.
Moreover, the dark current level and the white spot level of dark-time white spot also change depending on a shooting sensitivity. Here, the sensitivity depends on the system of the electronic camera (gain in amplifying a video signal). In other words, the higher the sensitivity is, the greater the dark current level and the white spot level of dark-time white spot become. Furthermore, the higher the sensitivity is, the larger the number of dark-time white spots gets. This is because the higher the sensitivity is, more easily the levels exceed the threshold value recognized as a dark-time white spot in the video signal of an electronic camera.
In the prior art, shooting is performed as follows to remove the influences of the dark current and the dark-time white spot.
That is, after normal shooting is performed, dark shooting is performed for the same length of time-in-second as that of the normal shooting. Then, the video signal obtained by the dark shooting is subtracted from the video signal obtained by the normal shooting to obtain a video signal from which the influence of the dark current and the dark-time white spot has been removed.
Incidentally, as a prior art relating to prevention of the dark current, there is one that reduces the amount of heat generated by an electronic camera to prevent the dark current (refer to Japanese Unexamined Patent Application Publication No. Hei 10-271398).
The above-mentioned prior art has the following problems.
Firstly, since dark shooting is performed after normal shooting of a subject is performed for the same length of time-in-second, it takes double the time of the normal shooting to complete shooting. In particular, when shooting of a celestial photograph etc., for example, requires 10-minute dark shooting after 10-minute normal shooting, and therefore it takes a long time (20 minutes or longer) until the shooting result can be visually recognized on a liquid crystal monitor.
Secondly, shooting twice doubles the power consumption.